Tracking down what seems to be an electrical issue

[b_raf]

A few months ago, I noticed that my oil temperature occasionally seemed high. Recently, during a flight, the temperature spiked to over 250°F. I decided to turn back to the airport, and during the steep turn, the temperature dropped to normal levels, only to spike back up once I leveled out.

After reading through some forums, I found that someone with the same avionics (AFS 3500) had experienced a similar issue. It was suggested they pull the alternator fuse to see if that was contributing to the problem. On my next flight, I tried this—pulled the fuse—and watched the oil temperature drop from 250°F to 170°F.

However, during that same flight, my CHT started reading high. Pulling the fuse caused the CHT to drop by a few hundred degrees, but it still showed elevated readings for the rest of the flight and taxi. When I toggled the master switch while shutting down, the CHT readings returned to normal.

I initially suspected a grounding issue but haven’t found any problems there. I’m wondering if anyone else has experienced this or has any insights into what might be causing it?

 

[Avanza]

A few months ago, I noticed that my oil temperature occasionally seemed high. Recently, during a flight, the temperature spiked to over 250°F. I decided to turn back to the airport, and during the steep turn, the temperature dropped to normal levels, only to spike back up once I leveled out.

After reading through some forums, I found that someone with the same avionics (AFS 3500) had experienced a similar issue. It was suggested they pull the alternator fuse to see if that was contributing to the problem. On my next flight, I tried this—pulled the fuse—and watched the oil temperature drop from 250°F to 170°F.

However, during that same flight, my CHT started reading high. Pulling the fuse caused the CHT to drop by a few hundred degrees, but it still showed elevated readings for the rest of the flight and taxi. When I toggled the master switch while shutting down, the CHT readings returned to normal.

I initially suspected a grounding issue but haven’t found any problems there. I’m wondering if anyone else has experienced this or has any insights into what might be causing it?

I would check if OIL, CHT transmitters require a GND on the engine and if so the engine GND-wire need to be checked.
Engine GND should be connected to firewall, battery negative and GND-bus.
The OIL and CHT transmitters use low voltage signals usually 0-5 V. Low voltage wire shoud not run parallel to the alternator wire or engine GND wire for any length.
It´s because large current in the alternator circuit may cause induction and hence false readings.
High oil temperature can also be the result of using wrong type of oil. Shell100 W straight oil has a low surounding air temperature limit of 61 F.
If surrounding air temp is 40 F, this may reduce oil flow in the oil cooler and result in high oil temperature readings.

Good luck.

 

[D-Dubya]

To borrow from the line on Anchorman, 90% of the time a bad ground will do that every time.

You can test for continuity and still not have a good ground. Those little buggers are tricky, especially if you're based in an environment near the coast or with high humidity--or both. Unbolt, unplug, or otherwise disconnect the affected grounds, then clean and re-attach them. You can even use a little dielectric compound or something like Corrosion X on the connection to prevent corrosion from forming.